<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>window.name - a</title>
</head>
<body>
    <iframe id="iframe" src="http://localhost:3001/c.html" frameborder="0"></iframe>
    <script>
        const iframe = document.querySelector("#iframe");
        let lock = false;
        iframe.addEventListener('load', function() {
            if(!lock) {
                lock = true;
                iframe.setAttribute("src", "http://localhost:3000/b.html");
            }else {
                console.log("page a.", iframe.contentWindow.name);
            }
        }, false);
    </script>
</body>
</html>